The use of an aftertreatment system to treat exhaust gas before the exhaust gas is exhausted to atmosphere is known. One known aftertreatment system uses a diesel oxidation catalyst (DOC) device that is intended to react with the exhaust gas to convert nitric oxide to nitrogen dioxide. In the case of diesel exhaust, a diesel particulate filter (DPF) can also be provided downstream of the DOC to physically remove soot or particulate matter from the exhaust flow.
When exhaust gas temperatures are sufficiently high, soot is continually removed from the DPF by oxidation of the soot. When the exhaust gas temperature is not sufficiently high, active regeneration is used. In the case of diesel engine exhaust, one form of active regeneration occurs by injecting fuel into the exhaust gas upstream of the DOC. The resulting chemical reaction between the fuel and the DOC raises the exhaust gas temperature high enough to oxidize the soot in the DPF.
A doser system that includes a doser injector is used to inject the fuel into the exhaust gas. Deterioration of the doser injector can occur over its lifetime, for example due to doser tip carboning or a reduction of doser stroke. Doser deterioration is believed to be one of the most frequent modes of failure in aftertreatment systems. A known doser monitoring method that attempts to determine the efficiency of the doser injector senses the temperature difference across the DOC. However, the effectiveness of this method is decreased by deterioration of the DOC which cannot be independently monitored.